Unagi Sushi
Freshwater eel

unagi sushi

What is Unagi ウナギ 【鰻】?

Unagi (ウナギ) is the Japanese word for the genus of freshwater eels and traditionally refers to the Japanese eel (jap. nihon-unagi). Unagi is a very popular ingredient in Japanese cuisine and is sold at a high price compared to other edible fish. In contrast to other fish species, the blood of the eel has a toxic effect and must therefore be heated sufficiently for consumption.

Unagi for Sushi and Sashimi

The mucous layer of the unagi can emit an unpleasant odour depending on the habitat and food. Therefore, both natural captive and farmed unagi are placed in clean water for one or two days before they are killed. The Japanese method of preparation of unagi follows several successive steps. First, the eel is cut open and filleted either at the head or on the ventral side, starting lengthwise. The meat is then cut into pieces of equal size, placed on skewers and gently roasted over charcoal. In the next step, the fillets are steamed and then dipped in sauce to be roasted again on the grill.

Ein Stück Aal-Sushi (als Nigiri geformt), dass mit Aal-Sauce (unagi no tare) bestrichen wurde und auf der Theke eines Sushi-Restraurants liegt.
Unagi Sushi (うなぎ寿司)

Unagi is rich in protein, vitamins and minerals. The cooked meat has an unmistakable taste which distinguishes it significantly from other fish. The prepared meat of unagi is very soft and fluffy, pleasant on the palate and usually without a fishy or earthy aftertaste. The high fat content gives it a full-bodied flavour, which can be enjoyed with a spicy soy-based sauce (jap. kabayaki) or sprinkled with salt alone (jap. shirayaki).

Occasionally the prepared meat from the unagi is sprinkled with Japanese pepper (jap. sansho, サンショウ) to give the meat a fresh smell. In addition, sansho is said to have a digestive effect which is supposed to support the absorption of the fatty unagi meat. For the preparation of unagi nigiri sushi, it is recommended to refrain from this and prefer sesame (jap. goma, ゴマ) or don't use any garnish at all.

Unagi Sauce (unagi no tare, うなぎのたれ)

Although unagi can be enjoyed without sauce (jap. tare), it is usually served with a thick broth specially prepared (jap. unagi no tare). At the time when the unagi sauce was not commercially produced in large quantities, it was considered an indication of the cook's skills. Nowadays, homemade unagi sauce is only to be found in high-quality or specialities restaurants.

The basic ingredients of unagi no tare are soy sauce, mirin, sugar and sake. Taste and consistency are similar to teriyaki sauce. Although unusual, the taste of the sauce is well suited as an addition to grilled meat dishes.

Unagi vs. Anago

Unagi is not considered a traditional ingredient for making sushi in Japan. Only rarely, or on advance order, you can find unagi as handmade nigiri in Japanese sushi restaurants (jap. sushi-ya, 寿司屋). More common is the use as an ingredient for pressed Sushi (oshi-zushi). In contrast, the conger eel (anago) is an essential part of the seasonal menu of a restaurant. Outside Japan, especially in North America and Europe, the situation is the opposite. Since Anago is rarely actively fished outside of Northeast Asia, it is simply too complicated or nearly impossible for most sushi chefs to obtain fresh Anago. Thus, mostly imported, frozen and (in the worst case) already industrially prepared unagi is used. Only in high-quality restaurants fresh and regionally available river eel is prepared.

Compared to anago, unagi is more tasty, fatty and meaty. Moreover, unagi is considered a higher quality delicacy in Japan and is therefore more expensive. In terms of taste, unagi differs from anago by its more intense and full-bodied taste. The common opinion is that anago, because of its lighter taste, harmonises better with soured sushi rice and is therefore the preferred choice for making sushi.

Best Season

Although unagi is traditionally eaten on the “day of the ox” (jap. doyō no ushinohi), late autumn until early winter is considered the best season. During this time, the fishes build up fat reserves in order to draw on it during the winter, when they hibernate in holes and hollows.

Even though unagi is most tasty towards the beginning of winter, unagi is traditionally appreciated in the middle of summer. In Japan they say unagi gives you strength and endurance, which you need especially during the Japanese summer heat to prevent exhaustion.

Wild Capture vs. Aquaculture

Wild-caught unagi (天然ウナギ) has a lighter and less fatty taste than farmed unagi (養殖ウナギ). The meat is harder and the animals are said to be slightly larger. In contrast, aquacultured unagi lacks the smell of mud and soil, has a softer texture and a more consistent taste.

Even if purists prefer wild-caught unagi, it can generally be said that a predominant proportion of consumers consider aquaculture reared unagi more tasty and of higher quality. According to the Food and Agriculture Organisation of the United Nations (FAO), in 2018, approximately 97 % of the unagi production volume came from aquaculture.

Chart showing the catch numbers of wild unagi and farmed unagi.

Unagi in Japan

Archaeological excavations from the Jomon period testify that unagi was already used as a source of food in ancient times. During the expansion of Edo, unagi was considered the food of the common workers, which they caught during the draining of Tokyo Bay. Until the beginning of the Meji period, unagi was sold in mobile food stalls (jap. yatai, 屋台) along the roadside. Nowadays, unagi is mainly found in speciality restaurants.

Unagi on the Day of the Ox (土用の丑の日)

In Japan it is a custom to eat tasty unagi on the “day of the ox” (doyō no ushinohi), a celebration day that is repeated annually at the end of July. The origin of this custom is not clear, the theories range from the mention in poems, to the invention by a writer from the Kyōhō period, to the theory that the Japanese characters for ox (うし) resemble two eels.

Unagi Trivia

In Japan, the term unagi nobori (うなぎのぼり), which can be freely translated as “climbing as an eel”, is similar to the expression “skyrocketing”. It means that the ascent or rise of something or someone progresses rapidly and is usually subject to special conditions and may only be temporary. The term is derived from the habit of unagi that after a certain time they return from the sea and hike up the mountain rivers to return to the ponds and lakes where they originally lived.


Unagi is a popular edible fish that has been fished in Japan and Europe for centuries and is valued as an ingredient. Growing consumer demand and the convergence of international trade flows in recent decades have increasingly contributed to its threat. The European eel is now considered to be in danger of extinction, while the Japanese and American eel, among others, are considered to be highly endangered. All species relevant as unagi are severely decimated in population size, although about 95 % of the consumed ungagi come from aquaculture, they are not bred in captivity. Instead, young eels (silver or glass eels) are taken from the wild and then reared in closed cultures and fattened until they are ready for slaughter.

Besides the excessive number of wild catches, the destruction of natural habitats, illegal fishing, and the fishing of juveniles for aquaculture poses an existential threat. Until the development of a so-called “complete aquaculture”, for which no glass eels have to be taken from the wild, a consumer must decide for himself to what extent each piece of unagi he orders affects the survival of the entire species.

Diagram showing the number of catches of wild unagi.
The worldwide demand for unagi is creating enormous pressure on natural populations stocks.


The common practice is to raise juvenile eels (glass eels) from wild stocks in aquaculture. It is still not possible to breed commercially viable unagi in captivity. Artificially fertilised eggs and successful hatching have been repeatedly achieved under laboratory conditions. Research continues to face serious problems in terms of food supply, resource consumption and the need for hormonal feminisation of the spawn.

Characteristics & Ecology

Freshwater eels (jap. unagi-ka) are aquatic and live in a variety of habitats, including freshwater, estuaries and marine areas. They spend most of their lives in rivers and migrate to the sea to breed.

The members of the unagi family have an elongated cylindrical body and many species reach a length of one meter. The largest species, the giant mottled eel (ōunagi), can reach a total length of up to two metres. Unagi are predominantly nocturnal predatory fish whose prey includes small fish, worms, crabs and also snails.

The economically most important species can be found in the following table.

Japanese nameCommon nameEndangerment (IUCN)
American eel
Anguilla rostrata
European eel
Anguilla anguilla
Critically endangered
Giant mottled eel
Anguilla marmorata
Least concern
Japanese eel
Anguilla japonica
Short-finned eel
Anguilla australis


Many fish belonging to the freshwater eel family (jap. unagi-ka) are considered commercially important seafood. In the last decade, the increase in industrial aquaculture breeding has shown enormous growth rates, with the largest production centres being located in East Asia.

Diagram showing the countries of origin of Unagi in 2018.
  1. Eel blood is toxic to humans and other mammals. It is a protein that denatures and loses its toxicity when heated at 60°C for more than 5 minutes, so that there is no danger when consuming completely cooked unagi.
  2. Analysis of Google search queries, as well as the evaluation of the advertisements of Japanese sushi chains, indicate an increasing demand for unagi nigri sushi in Japan.
  3. The holiday is taken from the Chinese zodiac calendar (jap. dasai kinen-hō, 太歳紀年法) and represents the midsummer time between two seasons.
  4. Since February 2013, the Japanese Ministry of Environment has officially listed the Japanese eel as a threatened species (MOE, 2013).
  5. In 2002, the Aquaculture Research Institute at the Fisheries Research Centre in Mie Prefecture succeeded for the first time in the world in developing larvae into glass eels (AFFRC, 2010).
  6. Among the main issues that researchers are facing is the need for shark eggs as a food source for juvenile eel immediately after hatching, the need for daily water changes and hormonal feminisation, as the vast majority of fish fry from artificial environments are male (FRANEWS, 2010).


© Eater. Chef Kanejiro Kanemoto Is Japan's Grilled Eel Master — Omakase. 2018-07-09, youTube.com



For copyright and author information, see the "Image Credits" section.


As a general rule, do not eat ingredients that are not explicitly labeled for raw consumption.

The blood of this seafood contains ichthyohemotoxin, which is toxic to humans and mammals. Consumption of larger amounts of blood can lead to death. In addition, contact with blood can trigger local inflammatory reactions, swelling or ocular sympathies, among other things. Caution is therefore required when handling raw meat. In order for the poison to lose its effect, the meat must be completely cooked at high temperature.

The use of unauthorized drugs or misuse of authorized drugs in seafood aquaculture poses a potential risk to human health. Only eat raw seafood from production facilities whose products are approved for raw consumption.

Seafood from particularly coastal or land-locked waters may be contaminated by varying amounts of industrial chemicals, including heavy metals and pesticides. These pollutants could accumulate in concentrations that, if consumed regularly, may cause health problems

References & Further Reading

  • [AFFRC, 2010]: The world's first 'complete eel farming' finally succeeds! ~Paving the way for eel production that does not depend on natural resources~, “世界初の「ウナギの完全養殖」、ついに成功!〜天然資源に依存しないウナギの生産に道を開く〜 ”, Press Release (プレスリリース). Fisheries Research and Education Center (水産総合研究センター), Agriculture, Forestry and Fisheries Research Council (AFFRC) (農林水産技術会議), affrc.go.jp. 2010. Retrieved online on: December 26, 2020
  • [FRANEWS, 2010]: Complete eel farming achieved (ウナギ 完全養殖達成). Fisheries Research Agency News, FRANEWS Vol. 23. Japan Fisheries Research and Education Agency (FRA) (国立研究開発法人水産研究). 2010
  • [MOE, 2013]: Publication of the Fourth Red List (Brackish and Freshwater Fishes) 第4次レッドリストの公表について(汽水・淡水魚類). Ministry of the Environment (MOE) (環境省), Tokyo (東京), env.go.jp. 2013
  • [Tsuji, 2012]: Tsuji Yasuhiro (辻泰弘編). A Taste of East and West - Comparing East and West by Cooking Methods - Eel (東西美味の品格-調理法で比べる東西の味わい-鰻). Shogakukan (小学館). 2012

Image Credits



Drawn illustration for  unagi

Common names

freshwater eel

Japanese names

  • mamushi (マムシ)
  • ujimaru (ウジマル)
  • unagi (ウナギ)

泉海魚, 鰻

Scientific name

Genus Anguilla


Freshwater eels


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